The Sun (also known as Kerbol) is the parent star of the planetary system in KSP, popularly referred to as the Kerbol System. As of version 0.18.2, five planets: Moho, Eve, Kerbin, Duna and Jool; and two dwarf planets: Dres and Eeloo; orbit around it. Kerbol is utterly massive containing 99.97% of the mass in the Kerbol system. This is very similar to our sun which contains 99.86% of the mass in the real world solar system.

"Kerbol" is a fan-made portmanteau of "Kerbal" and "Sol" — the Latin name for the real-life Sun. Before version 0.11, which introduced an object for KSP's parent star and an orbital Map view where it was explicitly named, it was an unnamed directional light source at infinite distance. Fans on the KSP forums invented and popularized the name, and the term "Kerbol System" was used in the official 1.2 release post, the first issue of "The Kerbal Chronicles" as well as in the official trailer for the KSP Enhanced Edition. It is however extremely important to note that the celestial body is called "The Sun" in the English game, and "Sun" in the game files. There is no such thing as Kerbol outside of the community forums and wiki.

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In-game Description

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The Sun is the most well known object in the daytime sky. Scientists have noted a particular burning sensation and potential loss of vision if it is stared at for long periods of time. This is especially important to keep in mind considering the effect shiny objects have on the average Kerbal.

Orbiting Kerbol

Escaping the SOI of any other body will place an object in orbit around Kerbol. This will give a Science situation of being in space high over Kerbol until an altitude less than 1,000 Mm is reached. Low orbit can be reached by using a standard Hohmann transfer from Kerbin's orbit, this requires about 8 km/s of change-in-velocity (Δv). The more efficient way to reach low Kerbol orbit, at the expense of drastically increased flight time, is to use a bi-elliptical transfer. With a sufficiently-distant apoapsis, the required Δv to "sundive" from Kerbin orbit can be reduced to under 4 km/s.

Previously the closest sustained altitude was 1,340 m, but now bodies emit radiation which heats parts and overheats parts before it could reach the atmosphere. The heating varies on the surface area it shines and part specific heat. Using parts with higher maximum temperature reduces the chance of overheating, making the inflatable heat shield a good choice to shield your delicate parts from heat. This imposes a challenge where the objective is to get as close to the Sun as possible while surviving.

The sphere of influence (SOI) of Kerbol is infinite, making Kerbol inescapable. Thus, achieving gravitation-free flight is impossible. With a powerful enough ship, it is possible to get to the point where the info tab will display the craft's situation as "escaping" Kerbol. The end of the patched conics path plotted by the game can be reached, at which point the spacecraft begins drawing a course behind it. But the actual escape never occurs. Presumably, this state would continue indefinitely, as the time required to find out what would happen would take several years real-time even at maximum time acceleration.

Landing

It's impossible to land on the Sun, even with the "Ignore max temperature" cheat enabled. Any objects are destroyed at an altitude below -250 m, which shows in the end flight dialog as "crashed into the surface of the sun" or similar, rather than ever landing.

One can attempt to place edited parts on the surface, but they bounce erratically at a few hundred to a few thousand meters per second if they do not explode.

Temperature and Luminosity

Temperature-height profile of Kerbol's atmosphere.

Kerbol's visible surface, its photosphere, has a temperature of 5840 K. This temperature, along with the star's yellow color, suggests that Kerbol is a Class G main-sequence star. Above the photosphere is a thin and hot atmosphere that is analogous to a chromosphere. The chromosphere extends to a height of 600 km and has at its base a pressure of 16 kPa. The temperature of the chromosphere decreases with increasing altitude until reaching a minimum of 4000 K at a height of 50 km. Above 50 km the temperature increases with increasing altitude. The maximum temperature of 10,000 K is reached at the atmosphere's extreme upper limit.

Based on its given size and temperature, Kerbol’s luminosity would be 56.7 yottawatts, however this is contradicted by other in-game physical parameters. The value of the solar constant at Kerbin is 1360 W/m2, which implies a solar luminosity of 3.16 yottawatts (0.008 Solar luminosities) and a surface temperature of 2840 K. These values are consistent with a Class M red dwarf.

Physical appearance

The equatorial radius of the Sun is almost four times than that of Jupiter and 43.6 times larger than Jool's, the largest planet currently known. Kerbol has the yellow color of a Class G main-sequence star and its absolute bolometric magnitude is approximately +10. If close enough, one can see that the dark spots are indented.

The "corona" surrounding Kerbol is not the actual size of the sun as it appears. It will shrink and grow relative to the location of the viewpoint, just as the size of Kerbol does normally. However, it won't shrink past a certain point, and if the sun is viewed from a far enough point, the corona is still very visible. This is apparent when using camera or telescope mods that remove the corona and show Kerbol's true size, which is usually much smaller than the corona appears.

In version 0.11, it became a yellow sphere which could be visited but its surface wasn't solid. It was named "The Sun" in Map view; however, fans had already taken to calling the star "Kerbol". Up to version 0.13.3 dropping below the "surface" was still safe, but a ship could break up from the extreme gravity gradients and/or be shot outwards at large fractions of the speed of light.

Reference Frames

Trivia

The name "Kerbol" was created by forum members and never appears in the game. Officially, the star is simply referred to as "The Sun", though the lead developer mentioned that its name could change once the need to differentiate it from other stars arises. The name "Kerbol solar system" is used to name the star system in the trailer for the enhanced console version of the game.

"Kerbol" is likely a portmanteau of "Kerbal" and "Sol", the Latin name of the Sun.

The Sun has just under twice the surface gravity of Kerbin, thus only slightly more than Eve's.

The Kerbal Sun is not a realistic analogue of our Real Sun.

The Mun is larger and closer to Kerbin than Moon to Earth, and thus in order to make eclipses work right, the Sun was made large enough to fit.

Real-world comparison

While otherwise analogous to our own real-life Sun, Kerbol has 1/113th the mass and 1/3rd its radius. Kerbol's other attributes can be used to derive the following statistics: it has a luminosity of approximately 3 yottawatts based on the solar constant at Kerbin; a defined surface temperature of 5840 K (that of a Class G star), but a temperature less than 3000 K based on its size and luminosity (that of a Class M star); the radius of a red dwarf; and the mass of a sub-brown dwarf. Such a star in real life would be on the limits of Hydrostatic equilibrium and its small size would mean it would only generate as much heat as a red dwarf. However, many astrophysical properties in KSP are effectively scaled down for gameplay.

It's real-life counterpart according to its size and mass would be 2MASS J0523-1403, the smallest star in existence.

Gallery

A picture taken of a probe on a close encounter with Kerbol shortly before overheating.